Papers by Author: Hajime Okumura

Abstract: We measure the temperature-dependent resistivity (ρ(T)) for thick heavily Al- and Ncodoped p-type 4H-SiC samples grown by chemical vapor deposition (CVD), physical vapor transport (PVT), and solution growth (SG), and investigate their conduction mechanisms. For samples with an Al concentration (CAl) of 3.5×10¹⁹ to 1×10²⁰ cm^-3 grown by CVD, PVT, and SG, the conduction mechanisms at high and low temperatures are band and nearest-neighbor hopping (NNH) conduction, respectively. In the range C_Al of 1×10¹⁹ to 3.5×10¹⁹ cm^-3, on the other hand, an anomalous conduction, referred to as X conduction here, is observed between the band and NNH conduction regions for the samples grown by CVD and PVT, but not those grown by SG. One of the differences between the samples grown by CVD and PVT and those grown by SG is the off-orientation toward [11-20] of the (0001) 4H-SiC substrate. We discuss the reason for the appearance of X conduction, which appears to be consistent with dopant-concentration inhomogeneity model.

Abstract: We evaluated crystalline quality of SiC p/n column layers over 20 μm thickness formed by trench-filling-epitaxial growth. Although threading dislocation density of trench-filling-epitaxial layer is almost same as flat n-type epitaxial layer, threading dislocations are localized in only trench-filled p-columns. We consider that threading dislocations migrated toward p-columns around trench bottom during trench-filling-epitaxial growth.

Abstract: We demonstrate 20 kV-class 4H-SiC n-channel implantation and epitaxial (IE)-IGBTs having both low on-state voltage and high blocking characteristics. We fabricated n-IE-IGBTs on a (0001) silicon face with free-standing epitaxial layers. Effective carrier lifetime increased significantly from 0.9 μs to 9.6 μs by a lifetime enhancement process. We used the IE structure to suppress an increase of the surface p⁺-well concentration, reduce implantation damage at the p⁺-well, and reduce junction field effect transistor (JFET) region resistance by ion implantation as a counter doping. The n-IE-IGBT at 100 A/cm² on-state voltage and specific differential on-resistance was 8.2 V and 36.9 mΩcm², respectively, at room temperature with a 30 V gate voltage. The blocking voltage was 26.8 kV at 45.7 μA.

Abstract: We investigate the temperature dependence of the resistivity and Hall coefficient for heavily Al-doped p-type 4H-SiC epilayers with Al concentrations (C_Al) of > 2E19 cm^−3, which are substrates for the collectors of insulated-gate bipolar transistors. The signs of the measured Hall co- efficients (R_H) changed from positive to negative at low temperatures. For epilayers with C_Al values of < 3E19 cm^−3, a negative R_H was observed in the hopping conduction region. In contrast, for epilayers with C_Al values of > 3E19 cm^−3, a negative R_H was observed in not only the hopping conduction region but also the band conduction region, which is a striking feature because the movement of free holes in the valence band should make R_H positive. For an epilayer with C_Al of 1.8E20 cm^−3, the sign of R_H clearly changed three times in the band conduction region. Moreover, the activation energies of the temperature-dependent R_H values were similar to those of the temperature-dependent resistivity in the corresponding temperature ranges, irrespective of the conduction mechanisms (band and hopping conduction).

Abstract: The scratch damage that caused the generation of double Shockley stacking faults (DSFs) in heavily nitrogen doped 4H-SiC crystal was investigated quantitatively. Scratch tests were carried out on 4H-SiC substrates with a nitrogen concentration of 2.6 × 10¹⁹ cm^-3. A residual tensile stress of 40 MPa was detected around the scratch loaded at 30 mN with a diamond tip. DSFs were generated from this scratch by annealing at 1100°C for 2 h in Ar atmosphere. After annealing, the residual stress around the scratch was reduced to a tensile stress of 10 MPa. This result suggests that the reduction of residual stress around the scratch coincided with the formation of DSFs.

Abstract: The temperature dependencies of the resistivity and Hall coefficient for heavily Al-doped 4H-SiC epilayers with Al concentration (C_Al) higher than 2×10¹⁹ cm^-3 were investigated. The signs of measured Hall coefficients (R_H) change from positive to negative at low temperatures. For the epilayers with C_Al < 3×10¹⁹ cm^-3 the sign inversion occurred in the hopping conduction region, which was reported to be explicable using the model for amorphous semiconductors. For the epilayers with C_Al > 3×10¹⁹ cm^-3, on the other hand, the sign inversion occurred in the band conduction region, which is a striking feature, because the movement of free holes in the valence band should make R_H positive. The sign-inversion temperature increased with increasing C_Al, while the dominant-conduction-mechanism-change temperature was almost independent of C_Al.

Abstract: The SiO₂/SiC interface quality has a significant effect on the performance of 4H-SiC MOS devices. The introduction of nitrogen to the SiO₂/SiC interface is a well-known method for reducing the interface state density (D_it). In this study, we introduced nitrogen to the SiO₂/SiC interface by forming SiN_x films using atomic layer deposition (ALD) and thus improved the interface quality. O₂ annealing with a SiN_x interface layer of optimal thickness enhanced the field effect mobility.

Abstract: By inspecting the CVD growth parameters, such as the flow rates of HCl and H₂ carrier gases, the pressure and the C/Si ratio, the trench filling in a high-rate mode with a high growth rate on the bottom and a relatively low growth rate on the mesa top was carried out. 4H-SiC trenches with the depths of 48 and 55 μm have been completely filled at the rates of 6.2 and 5.5 μm/h, respectively.

Abstract: To understand the effects of temperature and injection current density on expansion of Shockley stacking faults (SSFs) from basal-plane dislocations in 4H-SiC p-i-n diodes, the threshold current density for SSF expansion was investigated at eight temperatures by electroluminescence image observation. The threshold injection current density was found to decrease at lower temperatures and to increase at higher temperatures. We identified the origin of this temperature dependence and found that the limiting factor for expansion differed depending on the temperature.

Abstract: Trench-filling epitaxial growth of 4H-SiC by chemical vapor deposition (CVD) with and without HCl was analyzed based on a continuum-diffusion model including the Gibbs–Thomson effect. Qualitative reproduction of the reported observation showed that the effective surface free energy of SiC during CVD can be doubled by the addition of HCl